Abstract 2383 Structural analysis of the IRE1α-Sec61-Sec63 complex and its effects on IRE1α activity during ER stress

The inability of the rough Endoplasmic Reticulum (rER) to handle the incoming protein load leads to a buildup of unfolded or misfolded proteins and associated misfolding disorders. Upon reaching a set threshold, this buildup of unfolded proteins within the rough ER triggers the activation of the Unfolded Protein Response (UPR), which attempts to restore proper protein folding conditions. IRE1α, a key element of the UPR, is a membrane-bound quality control element which becomes activated in response to high volumes of unfolded proteins. Its activation involves auto-phosphorylation and oligomerization, leading IRE1α to splice a target RNA called XBP1. IRE1α removes a segment of the XBP1 messenger RNA, and the two ends are then ligated together by the RtcB tRNA Ligase. The resultant protein, XBP1s, acts as a transcription factor, inducing positive expression of various ER chaperones (including HSP40 family members B9, B11, and C3) as well as components of the Endoplasmic Reticulum Associated Degradation pathway (ERAD), which degrades messenger RNA to reduce ER protein load. It is known that IRE1α activity is attenuated when in complex with Binding immunoglobulin Protein (BiP), something that reduces XBP1u production during persistent ER stress. Recent studies have shown that formation of an IRE1α-Sec61-Sec63 complex induces IRE1α attenuation. Thus, it is suggested that BiP first binds to Sec63's J domain, then transfers to IRE1α, blocking its activity. Sec61 is theorized to act as a bridge between these two elements of the complex. The formation of this complex to attenuate IRE1α has been proven experimentally, but a structural investigation of this complex has yet to be performed. In this overview, we summarize the current knowledge and clinical significance of IRE1α, use molecular modeling techniques to investigate the structure and activity of IRE1α, and discuss its attenuation when in complex with Sec61 and Sec63. Our SMART Team, with support from 3-D Molecular Designs, has modeled the endoplasmic reticulum bound IRE1α protein, research of which could potentially lead to better treatment of diabetes and certain cancers.